1. Field of the Invention
The present invention relates to a robot system that performs a machining process on the workpiece by moving a machining tool and a workpiece relative to each other while pressing the machining tool against the workpiece with predetermined force.
2. Description of the Related Art
Known robot systems perform grinding or deburring of a work by using a robot. In such robot systems, the robot is moved while pressing a machining tool (a grinder, a sanding machine and the like) mounted at the wrist of the robot against the surface of the workpiece positioned in a movable range of the robot. Alternatively, the workpiece is held by the robot, and the robot is moved while pressing the workpiece against the machining tool fixed in the movable range of the robot. Meanwhile, an error may occur in the positioning of the workpiece or a holding position of the workpiece by the robot. Alternatively, an error may occur in a positional relation between the machining tool and the workpiece due to an individual difference of the workpiece, the wearing of the machining tool and the like. In order to prevent machining quality from deteriorating due to such an error, various methods are proposed.
JP2009-172692A discloses a method for teaching a motion trajectory of a robot which is provided with a compliance mechanism between a robot wrist and a grinding or deburring tool, so as to bring the tool into contact with a workpiece. According to the known art, even if there is an error in the positioning of the workpiece, an error in the holding position, or an individual difference in the shape or dimensions of the workpiece, it is still possible to absorb the error and appropriately perform the grinding or deburring of the workpiece, as long as the error is within a stroke range of the compliance mechanism.
U.S. Pat. No. 5,448,146 and JPH06-226671A disclose a method for teaching a motion trajectory of a robot which is provided with a force control mechanism including an actuator between a robot wrist and a grinding or deburring tool, so as to bring the tool into contact with a workpiece. According to the known art, even if there is an error in the positioning of the workpiece, an error in the holding position, or an individual difference in the shape or dimensions of the workpiece, contact force between the tool and the workpiece is controlled to be constant. Therefore, as long as the error is within a stroke range of the force control mechanism, it is possible to absorb the error caused by various factors.
JPS60-155356A and JPH08-087336A disclose a method for teaching a motion trajectory of a robot which is provided with a force sensor between a robot wrist and a grinding or deburring tool, so as to bring the tool into contact with a workpiece. According to the known art, force control is performed in a known manner, such as impedance control or hybrid control, and the robot is controlled such that contact force between the tool and the workpiece remains constant. According to the known art, it is possible to press the tool against the workpiece with predetermined force from any direction. Since a movable range of the robot is large, it is theoretically possible to absorb a large amount of errors.
JPH02-015956A discloses a method for teaching a motion trajectory of a robot which is provided with a compliance mechanism at a wrist, so as to bring a grinding or deburring tool into contact with a workpiece. According to the known art, reaction force received by the tool from the workpiece is measured, and the robot trajectory is corrected in real time such that the reaction force is a predetermined value.
JP2011-041992A discloses a method for adjusting, in a robot system provided with a force sensor and a hydraulic cylinder device, at least one of a target speed and a target track of a robot, target thrust of a hydraulic cylinder device, and a target motion speed of a machining tool in accordance with detection data of the force sensor.
In the method disclosed in JP2009-172692A, in the case in which a flexible unit is included in the compliance mechanism, the compliance mechanism is deformed due to gravity when the compliance mechanism is oriented in a horizontal direction or inclined with respect to a vertical direction. As a result, it is no longer possible to absorb the error in the positioning or undue pressing force may be generated. Furthermore, if the sum of the positioning error or the individual difference of the workpiece is larger than the stroke of the compliance mechanism, it is not possible to absorb the error. In the case where the compliance mechanism includes a spring, since the size of the positioning error and the pressing force are proportional to each other, the pressing force is changed from one place to another. Therefore, it cannot be ensured that machining quality is constant.
In the method disclosed in U.S. Pat. No. 5,448,146 and JPH06-226671A, even if the force control mechanism is oriented in the horizontal direction or is inclined with respect to the vertical direction, the mechanism unit is not deformed. However, similarly to the method disclosed in JP2009-172692A, it is not possible to absorb an error larger than the stroke of the compliance mechanism.
In the method disclosed in JPS60-155356A and JPH08-087336A, in which the pressing force is controlled through the operation of the robot body, responsiveness of the force control is affected by mass, inertia and rigidity of the robot body, and performance of an actuator for driving the robot, and the like. In general, as mass and inertia of the mechanism is larger, the responsiveness tends to be decreased. Accordingly, a large robot results in decreased responsiveness of the force control, and it may not be possible to achieve required machining quality. Similarly, also according to the method disclosed in JPH02-015956A, responsiveness of pressing force control is affected by mass, inertia and rigidity of the robot body and performance of an actuator for driving the robot, and the like.
According to the method disclosed in JP2011-041992A, in which a displacement amount of the hydraulic cylinder device is measured, when the position of the hydraulic cylinder device reaches a mechanical upper limit, the target track of the robot is revised. Therefore, it is also possible to absorb an error larger than the stroke of the hydraulic cylinder device. However, the track of the robot is revised only when the stroke reaches the upper limit. Therefore, there is a risk that the tool and the workpiece are temporarily not in contact with each other, or undue pressing force may be generated.
Thus, there is a need for a robot system which allows a machining process, such as deburring and grinding, to be appropriately performed, even if there is a positioning error, or an individual difference in the shape or dimensions of a workpiece.